Gas Works railways were once a common site throughout the country wherever coal was carbonised in retorts. The Birmingham Gasworks were originally horizontal retorts, but Saltley later had inclined retorts and then as the business was modernised the vertical retort became common. The locomotives were used for internal transport within the gasworks. Not all coals were suitable for carbonisation, although any coal could be used to heat the retorts. Gas coals arrived in railway wagons and where taken to the retort house by the private locomotives. Whilst much is known about these industrial locomotives, less is known about the system employed in the modern vertical retorts where the coke was taken away- there being some evidence to suggest light railways might have been used.